The Nature of Wind Noise: A Symphony of Aerodynamics and Mechanical Vibration
Wind noise from air conditioning fan motors is one of the most significant sources of noise during air conditioning system operation. It's not simply "wind noise," but rather a complex noise generated by the complex interaction of aerodynamics and mechanical vibrations. From a technical perspective, wind noise can be defined as the sound waves generated by the high-speed rotation of the fan impeller, which interacts with the air, causing airflow instability, turbulence, vortices, and pressure fluctuations. This noise is typically broadband, meaning that energy is distributed across a wide frequency range, but peaks occur at specific frequencies (such as the blade-passing frequency and its harmonics).
Sources of Wind Noise: Four Main Generating Mechanisms
1. Blade-Passing Frequency Noise:
This is the most representative component of wind noise. When fan blades rotate at high speed, periodically "cutting" through the air or fixed structures (such as the motor bracket and the volute tongue), they generate periodic airflow pulsations. This pulsation generates a specific frequency noise, known as the blade-passing frequency (BPF). The calculation formula is: BPF = number of blades × rotational speed (rpm). For example, a fan with seven blades and a rotational speed of 1200 rpm has a BPF of 7 × (1200/60) = 140 Hz. Due to varying sensitivity to specific frequencies, BPFs in the 1-4 kHz range can be particularly irritating.
2. Vortex Shedding Noise:
When air flows over irregular surfaces such as fan blades, brackets, and volutes, unstable vortices are formed. When these vortices break away from the surface, they generate random pressure fluctuations, creating a non-periodic, broadband noise. Vortex shedding noise often manifests as a hissing or whirring sound. It may not be noticeable at low wind speeds, but increases significantly at higher wind speeds. Controlling this noise requires optimizing the airflow path design to reduce unnecessary drag surfaces and sharp turns.
3. Turbulence Noise:
The rotation of the fan impeller creates a highly turbulent airflow. Turbulence itself is a random, disordered fluid motion containing vortices of varying sizes. The random motion and interaction of these vortices also generate broadband noise. Turbulence noise is proportional to the sixth power of wind speed, meaning that for every doubling of wind speed, the sound pressure level of turbulence noise increases by nearly 18 decibels. This is the primary reason why air conditioners experience a sharp increase in noise in "Power" mode.
4. Resonance Noise:
Resonance occurs when the natural frequency of the fan blades, volute, or the entire air conditioner structure is close to the noise frequency generated by the fan (such as the BPF). Resonance causes the vibration amplitude to increase dramatically, amplifying the originally subtle vibration noise into a loud sound. This noise often manifests as a "buzzing" or "roaring" sound, sometimes accompanied by perceptible vibrations. Controlling resonance noise requires optimizing structural materials, adding damping materials, or modifying the structural design to shift the resonant frequency.
Wind Noise Control Strategies: Comprehensive Optimization from Design to Application
To effectively reduce wind noise in air-conditioned fan motors, the industry has adopted a variety of technical measures, which are integrated throughout the entire product design, manufacturing, and installation process.
1. Impeller and Aerodynamic Design Optimization:
This is the key to fundamentally addressing wind noise. Through computational fluid dynamics (CFD) simulations, engineers can optimize the blade shape, curvature, pitch angle, and thickness to reduce airflow separation and turbulence, thereby reducing vortex noise. Furthermore, using unequal blade spacing or length can effectively disrupt the harmonics of the blower fan (BPF), dispersing its energy and reducing the sharpness of the noise.
2. Volute and Air Duct Structure Optimization:
The volute design is crucial to its impact on wind noise. Optimizing the spacing between the volute tongue and the impeller can reduce airflow pulsation during blade cutting. A streamlined volute inner wall and air duct design can reduce airflow resistance, turbulence, and vortices, thereby reducing noise. Some high-end air conditioners even employ bidirectional air intake or multi-layer duct designs to achieve smoother airflow.
3. Materials and Vibration and Noise Reduction Technologies:
Using polymer composite materials or sound-absorbing materials to manufacture the volute and duct effectively absorbs and attenuates sound waves. Using elastic vibration-damping pads or damping adhesive at the connection between the fan motor and the air conditioner casing can isolate motor vibration, preventing it from being transmitted through the structure to the air conditioner panel, thereby reducing structure-borne noise.
4. Motor Control Technology:
The use of variable frequency and brushless DC (BLDC) technologies is a trend in modern air conditioner fan motors. Because BLDC motors lack brushes, they operate more smoothly and quietly, and their speed can be precisely and continuously adjusted by a variable frequency controller. This allows the air conditioner to adjust the air speed according to actual needs. At low speeds, noise levels can be significantly reduced, effectively improving user comfort.
Wind Noise Measurement and Evaluation
Professionally, wind noise measurements are typically conducted in an anechoic chamber to ensure that the measurement results are not affected by external noise. Key measurement metrics include:
Sound Pressure Level (dB): This reflects the loudness of noise. A-weighted sound pressure level (dBA) is typically used because it more closely resembles the human ear's perception of loudness.
Sound Power Level (dB): This reflects the noise energy of the source itself. It is independent of the test environment and is the fundamental metric for evaluating a product's acoustic performance.
Spectral Analysis: By analyzing the distribution of noise across different frequencies, peak noise levels, such as blade cutting frequencies, can be identified, providing a basis for subsequent noise reduction design.
